1. Field of the Invention
The invention pertains to the extrusion of aluminum and to the construction of dies for such extrusion. Aluminum for extrusion purposes can be a commercial high purity grade of aluminum such as EC Alloy, Alloy 1050 or Alloy 1060 (Aluminum Association designations), all containing at least 99.45% by weight aluminum metal with less than 0.4% of alloying constituents such as silicon and iron and 0.05% or less of constituents such as copper, manganese, magnesium, and zinc. More commonly, extruded aluminum shapes are made from higher strength alloys containing 87-98% aluminum and varying small percentages of silicon, iron, copper, manganese, magnesium, chromium, zinc, and/or titanium, for example Alloy 2024, Alloy 5083, Alloy 5086, Alloy 5456, Alloy 6005, Alloy 6053, Alloy 6061, Alloy 6063, Alloy 6101, Alloy X6261, Alloy 6262, Alloy 6351, Alloy 6463, Alloy X7004, Alloy 7075, Alloy 7079, and Alloy 7104.
Detailed chemical compositions of these and similar alloys used for aluminum extrusion can be found in "Handbook of Aluminum"; Alcan Aluminum Corporation (third edition, 1970), pages 148-149. Correlation of these alloy number designation with ASTM specifications, SAE specifications, and federal specifications can be found in the same Handbook at pages 232-236. Correlation of Aluminum Association alloy number designations with designations used in Europe can be found in K. Laue and H. Stenger "Extrusion--Processes, Machinery, Tooling" (translated by A. F. Castle and G. Lang, American Society for Metals, 1981), at pages 128 and 438.
2. Description of the Related Art
In a typical extrusion, a preheated ingot or billet is placed in a hydraulic press and squeezed at high pressure through a steel die so that it emerges from the press in the desired cross section, which can be solid or hollow. Depending on the alloy used, the billet is preheated to a temperature in the range of 420.degree. to 500.degree. C. (in the lower part of the range for high aluminum alloys and higher for alloys with greater amounts of alloying constituents), and partly as a result of frictional heating at the die lands the extrudate exits the die at temperatures ranging from 560.degree. to 600.degree. C.
Traditionally, the extrusion dies for the extrusion of solid aluminum profiles having one or multiple outlets have been constructed with different bearing lengths according to the thickness of the profile and its distance from the center of the die as a way to regulate the flow of the metal and to obtain the correct shape and dimensions.
The bearing surfaces have a braking effect. As summarized by Laue and Stenger (at page 341), the length of the die lands should be at least twice the wall thickness for aluminum sections with a minimum value of 2.5 to 3 mm.
Feed prechambers have also been utilized to insure the coupling or welding between two consecutive extrusions, see Laue and Stenger at page 316.
U.S. Pat. No. 2,895,625 to Harris et al. disclosed a means to correct differences in the rate of flow of metal through different sections of the die orifice which comprises, in combination with such a die, a baffle mounted ahead of the inner face of the die and having an opening in line with and larger than that of each die orifice. The baffle plate is always oriented transversly to the flow of metal. There is no disclosure relating the thickness of the extruded product to the angle of entry of metal into the die.
U.S. Pat. No. 2,671,559 to Rosenkranz disclosed overcoming problems of cracks and fractures occurring under some conditions of operating an extruder, or forging press in his terms. Rosenkranz' solution is to make sure heterogeneous components of the substance of the grain boundaries which are soluble in the mixed crystal (as in alloys) are sufficiently softened but not fused; this is accomplished by operating in a temperature range higher than previously believed desirable. Maintaining the temperature in the desired range, according to Rosenkranz,
" . . . is dependent on, or made possible by, several factors namely the specific pressure of the forging press, the degree of pressing, the initial temperature of the billet, the pressing speed maintained during the pressing operation, and the conicity of the frictional surface of the die also. By using longer and more conical frictional surfaces of the die, more frictional heat is produced than by using short and non-conical pressing tools. (See column 4 lines 56 to 62 and line 73 to column 5 line 1.) Note the linkage of longer and more conical frictional surfaces to temperature control, which is also reiterated at column 5 lines 44 to 58." PA1 "The heat required for maintaining the temperature difference between the billet and the section is then produced by the heat of friction and deformation the magnitude of which is conditioned by the degree of deformation and the length and conicity of the frictional surface. According to recent discoveries by the inventor the maximal amount of frictional heat is obtained at angles of 2 to 60 between the frictional surface of the die and the direction of pressing. According to the cross sectional area of the section, however, often angles of inclination amounting at least to 1/20 and going up to 120 are possible." PA1 a die body having a prechamber formed therein with at least one step, a bearing length and at least one outlet communicating with the prechamber, the bearing length being constant throughout the profile, and an angle of entry regulating a flow of metal into the profile, the angle of entry being defined at each point of the profile as a function of the ratio of the perimeter to the thickness of the profile.
At the same time, the angle of inclination is here stated to be selected independently of the cross sectional area or thickness.
This is confirmed by the following two paragraphs (column 5 line 64 to column 6 line 24) and Table 1 included, where for the extrusion of a flat rod "the angle of inclination of the frictional surface must be smaller on the smaller side than on the wider side" but only the relative magnitudes of the angle are specified for various ratios of the length of the narrow side and broad side of a flat rod. Thus Rosenkranz shows the angle of inclination being varied while the thickness remains the same.
U.S. Pat. No. 5,095,734 to Asher disclosed an extrusion die including a "passageway with a bearing section having parallel bearing surface portions in the direction of extrusion which extend inwardly from an inlet opening a variable distance." Accordingly, the angle of entry is always the same, namely zero, and hence there can be no relationship whatever between the angle of entry and the thickness of the extrudate. Asher's summary of prior art includes drawings and discussion of extruder dies with tapered inlet portions, but no mention of any relationship between the angle of entry and the thickness of the extruded product.